uefi(data-types): allow is_ascii function on Char16 and CStr16

[RaitoBezarius]

Offers a way to know if a certain UTF-16 string contains only ASCII characters.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[phip1611]

Nice, thanks!

[phip1611]

Yikes, @RaitoBezarius . You put the checkmark to Update the changelog (if necessary) but you didn't actually added your changes to the changelog. Please have a look at it the next time :)

It would have been belonged to the "uefi - [Unreleased] / Added" section here: https://github.com/rust-osdev/uefi-rs/blob/main/uefi/CHANGELOG.md

[RaitoBezarius]

Yikes, @RaitoBezarius . You put the checkmark to Update the changelog (if necessary) but you didn't actually added your changes to the changelog. Please have a look at it the next time :)

It would have been belonged to the "uefi - [Unreleased] / Added" section here: main/uefi/CHANGELOG.md

Apologies, I read "if necessary" and thought that a simple add of function was not worthy of changelog. I will definitely ensure to do it next time (I hate when people do that too, so… my sincere apologies), may I recommend a PR (which I can send too) to change the template to make explicit what is necessary for a changelog update?

[phip1611]

Sure, go ahead. Thanks :)

Please keep it simple, short and precisely.

FYI @nicholasbishop